Pyxsteam

* fix issue with missing unit conversion in h_px and h_tx
* add tests for fixed bugs
* fix error in demo script
* add new demos based on the work of stu314159
* improve code readability by using additional tools
** enforce code styling with black
** reduce minimal python version to 3.4 based on report from vermin
** fix more spelling mistakes with codespell

* Fix issue 10: certain values lead to an infinite loop, stop if values dont change anymore and warn user
* Spelling

* Setup: add fallback to distutil if setuptools is not available

* removed stump for IAPWS R15 as it containd a error that need further work but left the constants define in R15
* non functional changes
** fix code style in multiple locations
** add github workflows for package and publish

* add functions from IAPWS R4 and R14
** R14: pressure along the melting and sublimation curves
** R4: viscosity and thermal conductivity as a function of density and temperature for heavy water substance
** test cases from documentation
** demo diagram
** add new constants

* fix bug in value check code in region_pT(p, T). Thanks to sbatururimi for reporting this!